RFR: CODETOOLS-7902967: jcstress: Rehash Unsafe usages in infrastructure and tests

[Aleksey Shipilev]

Currently we use `UnsafeHolder` in test infra and tests, and we can make those usages more straightforward.

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Commit messages:
 - CODETOOLS-7902967: jcstress: Rehash Unsafe usages in infrastructure and tests

Changes: https://git.openjdk.java.net/jcstress/pull/71/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jcstress&pr=71&range=00
  Issue: https://bugs.openjdk.java.net/browse/CODETOOLS-7902967
  Stats: 100 lines in 23 files changed: 26 ins; 4 del; 70 mod
  Patch: https://git.openjdk.java.net/jcstress/pull/71.diff
  Fetch: git fetch https://git.openjdk.java.net/jcstress pull/71/head:pull/71

PR: https://git.openjdk.java.net/jcstress/pull/71